Self-compensating thermocouple reading circuit

Abstract

A plurality of thermocouples are connected to reading circuit conductors at an isothermal junction block, for selective connection to the input of an operational amplifier. The resulting output signals from the operational amplifier are monitored and stored in a computer. A reference thermocouple is selectively connected to the operational amplifier through a reference junction compensator -- the reference junction compensator provides a compensated connection of the reference thermocouple with the reading circuit conductors. The signal at the output of the operational amplifier when the reference thermocouple is connected at the input is also monitored by the computer and stored for processing. In the computer, the signal sensed and stored when the reference thermocouple is connected to the operational amplifier is subtracted from the sensed, stored signals generated when the other thermocouples are connected to the operational amplifier. This subtraction nullifies the offset error of the operational amplifier (and the offset error of other reading circuit electronics such as the multiplexer selectively connecting the thermocouples to the operational amplifier), and it corrects the temperature indications of the thermocouples by eliminating the error of the cold junction at the isothermal junction block. These corrections are inherent, and are maintained without any surveillance or periodic adjustment.

I claim: 
     
       1. A self-compensating thermocouple reading circuit comprising: a first thermocouple adapted to sense a temperature to be monitored;   an isothermal junction block at which said first thermocouple is connected to reading circuit conductors;   a second thermocouple adapted to sense the temperature of said isothermal junction block;   a reference junction compensator connected to said second thermocouple to effect a compensated connection of said second thermocouple to reading circuit conductors;   an amplifier;   means for selectively connecting either said first thermocouple or said second thermocouple to said amplifier;   and means connected to the output of said amplifier to subtract the signal developed when said second thermocouple is connected to said amplifier from the signal developed when said first thermocouple is connected to said amplifier to thereby generate a signal proportional to the temperature to be monitored corrected for both the junction error at said isothermal junction block and the offset error of said amplifier.   
     
     
       2. A self-compensating thermocouple reading circuit comprising: a first thermocouple adapted to sense a temperature to be monitored;   an isothermal junction block at which said first thermocouple is connected to reading circuit conductors;   a second thermocouple adapted to sense the temperature of said isothermal junction block;   a reference junction compensator connected to said second thermocouple to effect a compensated connection of said second thermocouple to reading circuit conductors;   an amplifier;   a multiplexer for selectively connecting either said first thermocouple or said second thermocouple to said amplifier;   and means connected to the output of said amplifier to subtract the signal developed when said second thermocouple is connected to said amplifier from the signal developed when said first thermocouple is connected to said amplifier to thereby generate a signal proportional to the temperature to be monitored corrected for both the junction error at said isothermal junction block and the offset errors of said amplifier and said multiplexer.   
     
     
       3. A self-compensating thermocouple reading circuit comprising: a thermocouple adapted to sense a temperature to be monitored;   an operational amplifier;   connecting means for selectively connecting said thermocouple to said operational amplifier;   means for providing an input to said operational amplifier whereby the output of said operational amplifier reflects the offset error of said operational amplifier and a connecting error introduced by said connecting means upon connecting said thermocouple to said operational amplifier;   and means for subtracting the output of said operational amplifier reflecting the offset error and the connecting error from the output of said operational amplifier when said thermocouple is connected to the input thereof to thereby generate a signal proportional to the temperature at the junction of said thermocouple corrected for the offset error of said operational amplifier and the connecting error of the connecting means.   
     
     
       4. A self-compensating thermocouple reading circuit comprising: a plurality of thermocouples adapted to sense respective temperatures to be monitored;   an isothermal junction block at which said plurality of thermocouples are connected to reading circuit conductors;   a reference thermocouple adapted to sense the temperature of said isothermal junction block;   a reference junction compensator at which said reference thermocouple is connected to reading circuit conductors;   an amplifier;   means for selectively connecting each of said plurality of thermocouples and said reference thermocouple to said amplifier;   and means connected to the output of said amplifier for generating a plurality of signals respectively proportional to the temperatures at the junctions of said plurality of thermocouples corrected for both the cold junction error of said plurality of thermocouples at said isothermal junction block and the offset error of said amplifier.